spi: spi-fsl-dspi: Use non-coherent memory for DMA

Using coherent memory here isn't functionally necessary, we're only
either sending data to the device or reading from it. This means
explicit synchronizations are only required around those points and the
change is fairly trivial.

This gives us around a 10% increase in throughput for large DMA
transfers and no loss for small transfers.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-ID: <20250902-james-nxp-spi-dma-v6-4-f7aa2c5e56e2@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>